Ball Screw Selection for Thailand: RFP and FAT/SAT Guide

Ball screw selection is not simply a matter of choosing a diameter and lead from a catalogue. A machine may move yet still miss its positioning, service-life, thermal-stability or maintenance targets. For new equipment and retrofits in Thailand, the purchase specification must also address high ambient temperature, long operating hours, dust or chips, locally available lubricant, and the interfaces with the servo motor, support bearings and linear guides. This guide shows equipment buyers how to turn those requirements into comparable calculations, an RFP and measurable FAT/SAT acceptance criteria.

Start with operating conditions, not an accuracy grade

The correct sequence is application, load, stroke, speed and acceleration, duty cycle, orientation, accuracy, life, environment, maintenance and testing. Selecting a model first creates contradictions: a screw may meet travel speed but exceed critical speed; a high preload may provide stiffness but create thermal drift; a long calculated life may be defeated by contamination.

ISO 3408-1:2006 covers vocabulary and designation. Specify an accuracy grade together with the acceptance conditions and tests of ISO 3408-3 and the current accuracy standard and certificate of the chosen manufacturer. The grade alone does not guarantee machine accuracy. The screw, nut, support bearings, guides, mounting faces, frame, thermal behavior and control compensation must form one error budget.

How to cite the ISO 3408 series in a purchase specification

As of September 2026, ISO’s official pages describe Part 1 as vocabulary and designation, ISO 3408-2:2021 as nominal diameters, leads, nut dimensions and mounting bolts for the metric series, Part 3 as acceptance conditions and tests, Part 4 as static axial rigidity, and Part 5 as static/dynamic axial load ratings and operational life. ISO 3408-3:2006 was reconfirmed on 8 April 2026. ISO 3408-2:2021 reached “close of review” on 3 September 2026; do not infer the review’s final outcome from that status.

The ISO/TC 4/SC 11 page also lists ISO/TC 39/JWG 7 for revision of Parts 1 through 5. Therefore, do not write only “complies with ISO 3408.” Identify the part, edition, tolerance grade, inspection item and certificate. Define how a new edition issued during procurement will be handled through a documented review of scope, cost, schedule and retesting.

An eight-stage ball screw selection workflow

THK’s official selection inputs include mounting orientation, mass, guide type, friction, external axial force, target life, stroke, speed, acceleration/deceleration, reciprocations, positioning accuracy, repeatability, backlash, minimum feed and motor data. Convert them into eight procurement gates:

1. Define the function and worst credible load cases.
2. Select a preliminary lead and shaft diameter.
3. Verify axial load, buckling and permissible tension/compression.
4. Verify both critical speed and the DN-value limit.
5. Select accuracy, axial clearance, preload and stiffness.
6. Calculate L10 and convert it to time and travel.
7. Engineer thermal displacement, lubrication, contamination control and mounting.
8. Prove the integrated servo, coupling, bearings and axis in FAT/SAT.

Require every calculation to preserve inputs, units, load case, factors, catalogue revision, formula reference, margin and approver. A final spreadsheet cell without assumptions cannot be audited.

Diameter and lead: cycle time is not the only criterion

Lead is linear travel per screw revolution. A larger lead reduces rpm at a given linear speed, but changes required torque, minimum feed, resolution, stiffness and available nut designs. A smaller lead improves command resolution but raises rpm in a fast axis. A larger lead supports fast travel but demands a review of holding, braking and control resolution.

For a vertical axis, address falling risk during power loss or servo-off through the machine risk assessment. A ball screw can back-drive because of its high efficiency. Depending on the hazard, use a mechanical brake, counterbalance, fall-prevention device or monitored protective function rather than assuming the motor brake alone is sufficient.

Shaft diameter must satisfy more than dynamic load rating. Check buckling under maximum compression, permissible tensile/compressive load, critical speed, axial stiffness and feasible manufactured length. End support conditions and effective unsupported length must match the assembly drawing.

Permissible axial load and static safety factor

THK states that the screw shaft must not buckle under the maximum compressive axial load and that permissible tensile/compressive load related to shaft yield must also be considered. Its selection guidance gives lower-limit static safety factors against basic static load rating C0a of 2 without vibration or impact and 5 with vibration or impact. These are conditional maker guidelines, not universal guarantees. Starting, stopping, emergency deceleration, workpiece impact and changing process force must be classified honestly.

Maximum axial load is not merely mass multiplied by acceleration. Include guide resistance, process force, seal resistance, gravity on a vertical axis, preload and emergency-stop conditions. Keep maximum-load checks separate from equivalent-load life calculations. Ask the supplier to state which value was compared with C0a, permissible axial load, buckling load or tensile/compressive limit.

Permissible speed: use the lower of critical speed and DN limit

THK’s selection guidance determines permissible screw speed from shaft critical speed N1 and the DN-based permissible speed N2, using the lower value. Its critical-speed equation applies a 0.8 safety factor. DN limits vary by product and circulation design, so use the current data for the offered model instead of copying a single generic DN number into the RFP.

Peak rpm must reflect actual lead, reduction ratio, acceleration profile and possible overshoot. Even when the servo remains within its rated speed, the screw can exceed its critical or DN limit. Conversely, bearings, grease, seals, coupling or motor continuous limits may govern first. For the drive-system boundary, see the same-language guide to servo control versus inverter control.

Ball screw accuracy grade: derive it from the machine error budget

Do not automatically specify C3 for a machine tool or C7 for handling. Define absolute positioning, repeatability, reversal error, settling time, measuring point and thermal condition. Allocate the total allowance across lead accuracy, axial clearance, elastic displacement of screw/nut/bearings/mounts, guide attitude, frame deformation, thermal displacement, encoder/control effects and measurement uncertainty.

Axial clearance may have little effect in one-direction feed but appears as backlash when direction or load reverses. This is why preload, double nuts and offset-preload arrangements may be considered. A component certificate and a completed-machine measurement are different evidence. Require measurement length, reference temperature, travel error, variation, instrument and calibration, then confirm the assembled axis with a laser interferometer or equivalent method.

Preload: stiffness versus heat and life

THK explains that preload can reduce axial clearance below zero and improve rigidity under load reversal. Fixed-position approaches include double-nut and offset preload, while constant-pressure designs use a spring arrangement. Selection depends on stiffness, size, torque stability, temperature and application.

Preload also creates internal load. THK explicitly states that for a medium-preloaded ball screw, the applied preload must be considered in the life calculation because the nut already carries internal load. Higher preload can increase stiffness, but it also tends to raise running torque and heat and increase sensitivity to lubrication and misalignment. Specify the preload method, controlled preload value or method, dynamic torque, temperature rise, stiffness and calculated life—not merely “preloaded.”

During FAT, run the representative cycle until thermal behavior stabilizes. Record nut, bearing, motor and ambient temperatures together with position drift and drive torque. Excessive bearing preload or misalignment can create heat even when nut preload is correct.

Ball screw life calculation: L10 is not a warranty date

THK defines service life by rolling-fatigue flaking and nominal L10 life as the total revolutions that 90% of a group of identical ball screws can achieve under the same conditions without flaking. Basic dynamic load rating Ca uses a constant axial load corresponding to a rated life of 10^6 revolutions. L10 is neither a promise for an individual component nor a total-life model for poor lubrication, contamination, corrosion, installation error or accidental overload.

Calculate mean axial load from the actual sequence of load and speed, then apply justified load factors for vibration and impact. THK lists examples from 1–1.2 for very slight vibration/impact at V≤0.25 m/s, 1.2–1.5 for weak conditions at 0.252 m/s. This is not an automatic speed-only lookup; the physical process and mounting quality still matter.

Convert revolutions to L10h using operating rpm and real duty, or to travel using actual lead. Tell the supplier about short-stroke reciprocation because the same raceway region may be repeatedly loaded and lubricant distribution may differ. Instead of “five-year life,” state days per year, hours per day, cycles per minute, motion profile, load cases and maintenance assumptions.

Thermal displacement: one degree is not negligible

THK’s positioning guidance uses a thermal expansion coefficient of 12×10^-6/°C: a one-metre shaft expands by 12 µm for every 1°C rise. A 1.5 m effective length with a 5°C average rise therefore has a simple calculated expansion of 90 µm. Actual positioning error depends on the temperature distribution, support arrangement, constraints, frame and measurement point.

Heat comes from nut and bearing preload, lubrication drag, seals, speed, misalignment, motor and surrounding equipment. In Thailand, include normal air-conditioning operation, shutdown periods, day/night variation, enclosure doors and coolant temperature. Do not estimate shaft temperature from room temperature alone.

Controls can include reducing excessive preload, using a larger lead to reduce rpm, selecting the correct lubricant and quantity, oil/air cooling around the shaft, a negative target lead, tensioning the shaft where approved, linear-scale feedback and a controlled warm-up. THK gives a general example of a negative target value of -0.02 to -0.06 mm/m based on an assumed 2–5°C rise. It must not be copied without validation on the actual duty.

Lubrication, contamination and installation

Calculated life assumes sound lubrication and mounting. For chips, abrasive dust, welding spatter, wash-down or coolant, combine seals with bellows or telescopic covers, drainage and installation orientation. Confirm that covers do not bind or collapse at stroke limits.

The lubrication specification should include lubricant compatibility, viscosity range, initial fill, replenishment amount and interval, feed points, piping, purge method, mixing prohibition, storage and availability in Thailand. For automatic lubrication, test delivery, empty lines, blockage, alarms and remaining-volume monitoring. Excess lubricant can also increase drag and heat.

Record guide-to-screw parallelism, nut-housing position, bearing coaxiality, mounting-face geometry, tightening sequence/torque and full-stroke running torque. Do not use a flexible coupling to hide excessive alignment error.

Integrate the servo, gearbox and support bearings

Motor selection must include continuous and peak torque, speed, reflected inertia, acceleration, holding, regeneration and brake requirements. A gearbox may improve motor-side inertia matching or torque, but adds backlash, torsional compliance, heat and maintenance. Compare direct drive and reduction using the same lead, speed, resolution, cycle and reversal requirements; the gearbox reducer selection guide provides the wider trade-off.

Support bearings determine axial stiffness and running accuracy. Specify fixed-side angular-bearing arrangement and preload, locknut assembly, supported-side thermal freedom, grease, seals and replacement method. In a fixed-fixed arrangement, verify the axial-force change created by temperature.

RFP checklist for ball screw procurement

Scope and operation

• Axis, process, workpiece and equipment ID
• Supply boundary: screw, nut, bearings, coupling, motor, cover and lubrication
• Horizontal/vertical orientation, stroke, effective thread length and overall envelope
• Peak/continuous speed, acceleration, deceleration, jerk and settling time
• Duty diagram, annual hours, mass, external/process force and emergency stop

Performance and calculations

• Absolute accuracy, repeatability, reversal error and minimum feed
• Measurement point, direction, temperature, warm-up and load
• C0a static safety factor, buckling and tensile/compressive limit
• Critical speed, DN limit and permissible rpm
• Ca, mean load, load factor, preload and L10/L10h
• Stiffness, elastic displacement, thermal displacement and motor sizing

Product and evidence

• Ground/rolled construction, diameter, lead, grade and preload method
• Nut/circulation type, seals, covers, support bearings and coupling
• Lubricant, quantity, interval, delivery system and local availability
• Applicable standards and editions, catalogue revision, drawings, bill of materials and inspection certificates
• Advance approval for changes to model, maker, grade, preload, lubricant, seals or bearings

“Equivalent” is not an acceptance criterion. A change request must quantify differences in dimensions, load, life, speed, stiffness, heat, torque, maintenance, delivery and retesting.

FAT and SAT: prove the design with evidence

Separate mechanical results before compensation from compensated results. SAT then repeats the necessary checks after transport, installation, foundation fastening and connection to real utilities and surrounding equipment. Use real workpieces, cycle time, cold-to-warm drift, plant temperature range, interlocks, recovery, local lubricant/spares and continuous operation.

If the retrofit changes controls, align the ball-screw tests with the PLC replacement decision guide and control panel design guide so I/O, interlocks, backups, rollback and FAT/SAT ownership are consistent.

Compare quotations by assumptions, not headline price

Cost changes with diameter, length, lead, accuracy, preload, shaft-end machining, bearings, covers, lubrication, quantity and lead time. There is no verified universal Thai market price that should be quoted as fact. Compare the same work breakdown: component, calculations, drawings, machining, assembly, measurement, FAT, installation, readjustment, spares, transport, warranty and training.

A rolled screw is not automatically inferior, and a ground precision screw is not automatically the correct choice. Use the lowest specification that satisfies the documented duty and acceptance evidence. Require every bidder to list assumptions, exclusions, unknowns and alternatives.

Summary: connect selection calculations to acceptance evidence

Good ball screw selection is not defined by a maker name or accuracy grade. Freeze operating inputs; verify maximum axial load, static safety, buckling, the lower of critical-speed and DN limits, life including preload, stiffness, thermal displacement, lubrication, contamination and mounting; then carry those same requirements into FAT and SAT. Values such as 12 µm/m/°C thermal expansion and static safety-factor guidance of 2 or 5 are useful only with their conditions and sources. Continue to monitor the ISO 3408 revision work and identify the applicable part and edition in the contract.

TOMAS TECH can support Thailand projects before a part number has been selected—from load/speed/accuracy inputs and supplier calculation review to RFP and FAT/SAT criteria. Contact us while the machine or retrofit specification is still being defined.

Ball screw selection FAQ

Should I choose C3 or C5 accuracy?

Do not choose by application name alone. Quantify absolute position, repeatability, reversal, settling and temperature, allocate an error budget and confirm the assembled axis by measurement.

Is higher ball screw preload always better?

No. Preload reduces clearance and raises stiffness, but also adds internal load, torque and heat and must be included in life calculations. Select it from the required stiffness and reversal performance, then verify temperature and drift.

Is L10 the replacement date?

No. L10 is a rolling-fatigue statistic: 90% of an identical group under identical conditions reaches the stated revolutions without flaking. Build a separate maintenance plan for lubrication, contamination, corrosion, installation and covers.

Can servo compensation solve thermal displacement?

It helps only when the temperature field and deformation are repeatable. Reduce heat first, measure screw/nut/bearing and ambient temperature, standardize warm-up and validate the model. Consider external linear feedback for high accuracy.

What calculations should a ball screw RFP require?

Maximum and mean axial load, C0a static safety, buckling, tensile/compressive limit, critical speed, DN limit, Ca and preload-adjusted life, axial stiffness/displacement, thermal displacement, and motor speed/torque/inertia—with inputs and source revisions.

How should FAT and SAT be divided?

FAT checks documents, assembly, no-load/loaded performance, accuracy, thermal stability, faults and maintainability before shipment. SAT repeats site-dependent checks under real temperature, workpiece, duty, interfaces and continuous production.

References

1. ISO 3408-1:2006
2. ISO 3408-2:2021
3. ISO 3408-3:2006 — Acceptance conditions and acceptance tests
4. ISO/TC 4/SC 11 — ISO 3408 revision JWG
5. ISO 3408-4:2006 — Static axial rigidity
6. THK — Conditions of the Ball Screw
7. THK — Permissible Axial Load
8. THK — Permissible Rotational Speed
9. THK — Selecting a Nut
10. THK — Static Safety Factor
11. THK — Studying the Service Life
12. THK — Positioning Accuracy and Thermal Displacement
13. NSK — Identify Ball Screw Quality Before It’s Too Late